Temporal bone:Facial nerve
The facial nerve carries sensory, secremotor and motor fibres to the face. It emerges at the lower border of the pons above the olive. Three brainstem nuclei contribute to the facial nerve: one motor, one secretomotor, and one sensory: * facial nerve motor nucleus in the pons: as the motor fibres of the facial nerve loop posteriorly over the abducens nerve nucleus, they form the facial colliculus in the floor of fourth ventricle * superficial salivary nucleus: next to the facial nucleus supplies secretomotor parasympathetic fibres (to submandibular, sublingual, lacrimal glands) as nervus intermedius * nucleus of tractus solitarius: lateral to the dorsal nucleus of the vagus nerve, supplies taste fibres that eventually end up in the chorda tympani, 2/3 of tongue. Fibres travel with nervus intermedius. General course The facial nerve has six named segments: # intracranial (cisternal) segment # meatal segment (internal auditory canal): 8 mm - zero branches # labyrinthine segment (IAC to geniculate ganglion): 3-4 mm, 3 branches (fromgeniculate ganglion) # tympanic segment (from geniculate ganglion to pyramidal eminence): 8-11 mm, zero branches # mastoid segment (from pyramidal eminence to stylomastoid foramen): 8-14 mm, 3 branches # extratemporal segment (from stylomastoid foramen to division into major branches): 15-20 mm, 9 branches Intracranial (cisternal) segment The nerve emerges immediately beneath the pons, lateral to the abducens nerve and medial to the vestibulocochlear nerve and is joined by the nervus intermedius (from CN IX), which has emerges between the two (hence name). Together the two travel through the cerebellopontine cistern to the internal acoustic meatus along with the vestibulocochlear nerve. Meatal segment The nerve pass in the petrous bone in the anterosuperior quadrant of the fundus of the meatus. Labyrinthine segment The facial nerve passes through the internal acoustic meatus anterolaterally between and superior to the cochlea (anterior) and vestibule (posterior). Near the middle ear, the nerve makes a sharp 90 degree bend (1st genu) and forms the geniculate ganglion (where the nervus intermedius joins the facial nerve and where fibers for taste synapse). Three branches originate here: * greater superficial petrosal nerve: consist of nervus intermedius. It passes forwards through the petrous bone and reach the trigeminal ganglion. At the foramen lacerum, it is joined by the deep petrosal (sympathetic nerve from the ICA) and enter the pterygoid canal as the vidian nerve. It enters the pterygopalatine ganglion. Here secremotor is relayed to nose, pharynx, sinuses and lacrimal fibres join the maxillary artery and enter the orbit as the zygomatic branch. * lesser petrosal nerve * external petrosal nerve The labyrinthine segment is the shortest only measuring 3-4 mm. It is also the narrowest and the most susceptible to vascular compromise. Tympanic segment As the nerve passes posteriorly from the geniculate ganglion it becomes the tympanic segment (8-11 mm in length) and is immediately beneath the lateral semicircular canal. It turns inferiorly 90 degree (2nd genu) to run down the medial wall of the aditus to antrum, posterior to tensor tympani and oval window. Just distal to the pyramidal eminence the nerve makes a second turn passing vertically downwards as the mastoid segment. The tympanic segment has no branches. Mastoid segment The mastoid segment, measuring 8-14mm in length, extends from the second genu to the stylomastoid foramen, in the facial canal. It gives off three branches: * nerve to stapedius: reach the muscle by a minute canaliculus. * chorda tympani: terminal branch of the nervus intermedius. It runs past the posterior wall of the middle ear, between malleus and incus, and then anteriorly across the tympanic membrane to the anterior wall and leaves though the petrotympanic fissure. It joins the lingual nerve in the infratemporal fossa. * nerve from the auricular branch of the vagus nerve (CN X): pain fibers to the posterior part of the external acoustic meatus hitchhike from the jugular foramen Extratemporal segment As the nerve exits the stylomastoid foramen, it gives off posterior auricular branch. It then passes between the posterior belly of the digastric muscle and the stylohyoid muscle (supplying both) and enters the parotid gland posteromedially. Lying between the deep and superficial lobes of the gland the nerve divides into a superior temporofacial and and inferior cervicofacial branches. These branches forms a plexus called pes anserinus, and is superficial to the retromandibular vein and ECA. Finally from the anterior border of the gland, five branches emerge; temporal, zygomatic, buccal, mandibular (marginal) and cervical. * Temporal: emerge from upper border and cross over the zygomatic arch. It supplies auricular muscles, part of frontalis. * Zygomatic: across the zygomatic arch and zygomatic bone. Supply the orbicularis oculi. * Buccal: run forwards close to the parotid duct. Supply the buccinator, muscles of the nose and upper lip. * Marginal: runs along the lower border of the mandible, reaching the anterior border of the masseter and passing superficial to facial artery and vein. Supplies the mouth depressors (e.g. depressor anguli oris) * Cervical: pass downwards from lower border of parotid and supplies platysma. Function Taste Taste fibers to the anterior two thirds of the tongue originate in the nucleus of the tractus solitarius (NTS), travel in the nervus intermedius (preganglionic) where they join the facial nerve at the geniculate ganglion and synpase. Postganglionic fibers travel with the facial nerve and are given off as the chorda tympani, which eventually joins thelingual nerve (branch of the trigeminal nerve (CN V)). Parasympathetic Preganglionic fibres originate in the superior salivary nucleus and join the facial nerve at the geniculate ganglion having traveled with the nervus intermedius. They do not synapse in the ganglion, but rather pass through to be distributed between: # greater superficial petrosal nerve anastamosing in the pterygopalatine ganglion and supplying the lacrimal gland # lesser petrosal nerve anastamosing in the otic ganglion and supplying the parotid gland (along with fibers from the inferior salivary nucleus which arrive via the glossopharyngeal nerve (CN IX) supply to the tympanic plexus) # chorda tympani anastamosing in the submandibular ganglion and supplying the submandibular gland and sublingual gland Blood supply The facial nerve receives its arterial supply from three main sources: # labyrinthine artery: a branch of the AICA, supplies the meatal segment. It is supplemented by direct twigs in the cisternal position, directly from the AICA # superficial petrosal artery: a branch of the middle meningeal artery which passes retrogradely along the greater superficial petrosal nerve and also supply geniculate ganglion. # stylomastoid artery: a branch of the occipital artery, which pases retrogradely into the stylomastoid foramen and supply mastoid and tympanic segments. The labyrinthine segment is the most vulnerable to ischaemia as the connections between the labyrinthine artery and superficial petrosal artery are very tenuous, with each artery essentially being end arteries.